The invention relates to a computer system that is connected to a data communications network, e.g., the Internet or an Intranet. Such computer systems, which are typically individual computers, e.g., PCs, are increasingly exposed to computer viruses and unauthorized access to internal data. As a rule, virus scanners can detect and eliminate only known computer viruses but not viruses that are completely new and that have entirely different structures than previously known viruses. In particular, in the case of computers in administrative offices, banks, insurance companies, and in industry (e.g., for operating and monitoring automation systems), which increasingly communicate with other computer systems, e.g., control centers, via public data communications networks, infection with computer viruses can cause enormous damage. For example, programs called Trojan horses can infiltrate the computer masquerading as a benign application, and can secretly spy out internal data and transmit this data to an external location.